1. Field of the Invention
This invention relates to a method for recovering autorefrigerative and ketone solvents while preventing water buildup in the recovered solvent in a solvent dewaxing process. More particularly, this invention prevents water buildup in a process for recovering autorefrigerative/ketone dewaxing solvents by passing same to a decanter and deketonizer, the excess water being removed therefrom in the deketonizer. Still more particularly, this invention relates to an improved process for preventing water from building up in the recovered solvents of an autorefrigerative/ketone solvent dewaxing process which employs steam stripping to remove at least a portion of the solvents from the wax or dewaxed oil, by passing the wet overheads from the stripping step to a decanter and deketonizer, excess water being removed from the wet solvent in the deketonizer.
2. Description of the Prior Art
It is well known in the art to remove waxy constituents from wax-containing hydrocarbons, particularly from wax-containing petroleum oils such as lube oil stocks, by various methods. Generally these processes comprise mixing the waxy oil with a solvent and chilling the resultant mixture to a temperature at which the wax crystallizes out of solution. The amount of wax removed from the oil depends on the type of oil to be dewaxed, the amount and composition of solvent used, the temperature at which the separation of dewaxed oil from precipitated wax takes place, etc.
A multitude of so-called dewaxing processes have been developed and are well known in the art. These include both batch and continuous dewaxing processes. Many processes have also been developed around the application of a particular type of dewaxing solvent. Two types of solvent dewaxing processes have gained particularly widespread use in the art. These are known as the ketone dewaxing process and the autorefrigerative (propane, propylene, etc.) dewaxing process. More recently, a process employing a dual-solvent system comprising a mixture of an autorefrigerant with a wax antisolvent such as a ketone has been developed and commercially tested.
Solvents employed in ketone dewaxing processes are generally ketones containing from three to six carbon atmos such as acetone, methyl ethyl ketone, methyl isobutyl ketone, etc. or mixtures thereof and they may also be used in combination with aromatics solvents such as benzene, toluene, petroleum naphtha or mixtures thereof. Chilling is accomplished either indirectly in scraped surface heat exchangers after the solvent has been thoroughly mixed with the waxy oil, or, mixing and chilling are accomplished simultaneously by incrementally injecting the cold dewaxing solvent into the waxy oil along a plurality of points or stages along a cooling tower as the waxy oil passes through same, which is known in the art as DILCHILL crystallization, or, DILCHILL crystallization followed by additional cooling in scraped surface heat exchangers. A major disadvantage in all of the ketone dewaxing processes is the need for heat exchangers to cool the dewaxing solvent, the scraped surface devices, etc.
Autorefrigerative dewaxing processes employ a low molecular weight autorefrigerant hydrocarbon, such as propane, which is a gas at standard temperatures and pressures. The autorefrigerant is added to the warm, waxy oil as a liquid under pressure. It is then allowed to evaporate, thereby autorefrigerating the mixture in situ. The disadvantage of this process as compared with the ketone processes is that the relatively high solubility of wax in the autorefrigerant at any given temperature does not permit the removal of as much wax as, or of producing dewaxed oil with pour points as low as, is achieved with the ketone dewaxing processes at the same filtration temperature. This requires that the oil be chilled to substantially lower temperatures when employing autorefrigerative solvents for in situ chilling than in ketone dewaxing processes, in order to achieve a specified wax content or pour point.
A great improvement in dewaxing processes has been realized in recent years by the use of an autorefrigerant/ketone dual-solvent system. Basically, this dual-solvent process employs a highly volatile autorefrigerant such as propane or especially propylene, in admixture with a ketone wax antisolvent, such as acetone or methyl ethyl ketone, in order to reduce the solubility of the wax in the autorefrigerant. One such process is taught in U.S. Pat. No. 3,503,870, the disclosures of which are incorporated herein by reference. This dual-solvent system combines most of the advantages of both the autorefrigerant and the ketone dewaxing processes. Low pour point oils can be obtained with only a small difference between the wax filtration temperature and the pour point of the dewaxed oil. At the same time, autorefrigeration is used for in situ cooling thereby eliminating or reducing the necessity of scraped surface heat exchangers. Although the dual-solvent system has many advantages, one of its major disadvantages has been the lack of an efficient solvent recovery process that prevents water buildup in the solvent system.
Solvent recovery systems for dual-solvent dewaxing processes have been directed towards recovering the autorefrigerative and ketone solvents from the dewaxed oil and wax through the use of high and low pressure flash evaporation followed by gas stripping, as outlined in U.S. Pat. No. 3,622,496, the disclosures of which are incorporated herein by reference. U.S. Pat. No. 3,622,496 is directed toward the use of autorefrigerant vapors for stripping the remaining solvent from the dewaxed oil and wax after most of it has been recovered by a combination of high and low pressure flash evaporation. This system, using gases such as propane or propylene, attempts to alleviate the water problem by recycling dry gas instead of employing once through steam for final removal solvent. However, there are two disadvantages to this system. The first is that some water always enters the process dissolved in the feed with a resultant water buildup in the solvent. The second disadvantage is that some propylene dissolves in the liquid leaving the low pressure flash towers resulting in loss of solvent and also requiring explosivity corrections to the dewaxed oil and wax products to remove the last traces of dissolved propane or propylene.
It would be a significant improvement to the art if steam stripping could be efficiently and economically employed to remove the last traces of solvent from both the dewaxed oil and wax without causing water buildup in the solvent system. If too much water is allowed to build up in the solvent system of a dual-solvent dewaxing process, ice crystals form on the wax cake in the wax filters clogging same, thereby upsetting the filtration operation. More importantly, when a portion of the solvent is cooled to filtration temperature for use as wash and reslurry solvent, ice crystals tend to plug up the heat exchangers in which it is cooled, as well as the control valves and small passages in the headers which distribute it over the filter drums in the wax filters.